The present invention relates to a pretensioner for a seat belt system and, in particular, to a pretensioner in which a seat belt take-up shaft is rotated by a rotary actuator operated by gas pressure.
Pretensioners are increasingly being provided in the seat belt systems of vehicles to tighten the belt, which is loosely held in engagement with the vehicle occupant, in an emergency such as vehicle collision and to restrain the body of the occupant more effectively. Various types of operating mechanisms for pretensioners are known. One type of mechanism has a rotary actuator operated by gas pressure in which a belt take-up shaft or a take-up reel is rotated by a rotating member of the actuator. Pretensioners of that type are disclosed in U.S. Pat. No. 4,455,000 ("Ref. 1"), Japanese Patent Laid-Open Publication No. 58-195571 (corresponding to U.S. Pat. No. 4,588,832, "Ref. 2"), Japanese Patent Publication No. 59-15657 ("Ref. 3"), and Japanese Patent Laid-Open Publication No. 60-45449 ("Ref. 4").
When a seat belt retractor is used for a take-up mechanism for pretensioning the belt, the pretensioner must not interfere with the normal belt-winding and pull-out operation of the retractor. Thus, the rotating member of the pretensioner actuator is separated from the seat belt take-up shaft or take-up reel when not in operation, and is engaged with them only during operation.
A clutch mechanism is needed for the separation, but in each of the references referred to above, the clutch mechanism is complicated and costly, such as the arrangements of Ref. 1, 2 and 4. Moreover, a take-up shaft of greater length is required, because the clutch mechanism is arranged in series with the rotating member in the axial direction of the take-up shaft. In Ref. 3, a more reasonable arrangement, in which a pin supported on a vane is operated by gas pressure, is proposed. However, since the engaging part of the pin is received in an axial hole in a flange of the take-up reel, there is some restriction to the alignment pitch of the hole with respect to hole diameter, and the device cannot be made to have an accurate and precise operation. this leads to the problem that there is undesirable play with respect to when engagement occurs. Also, because the engagement of the pin with the hole is only achievable after the vane is rotated, the engaging operation is not consistent.